The distributions of howling monkeys (Alouatta pigra and A. palliata) in southeastern Mexico and Central America.

We compared the distributions of Alouatta palliata and A. pigra in southeastern Mexico and Central America with geographic and ecological features to infer current barriers and ecological preferences. Distribution data were obtained from museum specimen localities, study sites, historic records and field surveys and integrated into digital elevation and ecosystem maps using GIS. A. pigra evidently occurs at a number of sites above 2,000 m, where temperatures can even drop below zero on some days of the year, thus indicating a broader ecological tolerance than previously reported. Both species occupy a number of vegetation types and can be found in seasonal and nonseasonal forests. We identified the highland massif of northern Central America and its associated coniferous and subalpine vegetation as a geographic barrier that separates the species. In the past, distribution maps for these species have indicated adjacent contiguous ranges, but we propose that they are largely separated by these mountains. There are two contact zones: a broad area of sympatry north of the highland massif in Mexico and a narrow zone in eastern Guatemala where parapatry is maintained by a river barrier and where only A. pigra occurs in the high elevations and cooler habitats inland. We explore an alternative biogeographic scenario for the split of the two species that accounts for the current distribution and differences in elevation and cold tolerances.